In 1911 Matthews said he had invented an Aerophone device, a radiotelephone, and transmitted messages between a ground station and an aeroplane from a distance of two miles. His experiments attracted government attention and on 4 July 1912 he visited Buckingham Palace. However, when the British Admiralty requested a demonstration of the Aerophone, Matthews demanded that no experts be present at the scene. When four of the observers dismantled part of the apparatus before the demonstration began and took notes, Matthews cancelled the demonstration and drove observers away.

Newspapers rushed to Matthews's defence. The War Office denied any tampering and claimed that the demonstration was a failure. The government later stated that the affair was just a misunderstanding.

In 1914, after the outbreak of the First World War, the British government announced an award of £25,000 to anyone who could create a weapon against zeppelins or remotely control unmanned vehicles. Matthews claimed that he had created a remote control system that used selenium cells. He successfully demonstrated it with a remotely controlled boat to representatives of the Admiralty at Richmond Park's Penn Pond. He received his £25,000 but the admiralty never used the invention.

Next, Matthews appeared in public in 1921 and claimed to have invented the world's first talking picture, a farewell interview of Ernest Shackleton recorded on 16 September 1921, shortly before Shakleton's last expedition. The film was not commercially successful. Other talking-picture processes had been developed before that of Matthews, including processes by William K. L. Dickson, Photokinema (Orlando Kellum), and Phonofilm (Lee DeForest). However, Matthews claimed his process was the first sound-on-film process. Even if Matthews's process actually worked, it was certainly not the first.

1925 photo that purports to show a night demonstration of the ray on the island of Flat Holm

In 1923 Matthews claimed that he had invented an electric ray that would put magnetos out of action.[3] In a demonstration to some select journalists he stopped a motorcycle engine from a distance. He also claimed that with enough power he could shoot down aeroplanes, explode gunpowder, stop ships and incapacitate infantry from the distance of four miles. Newspapers obliged by publishing sensational accounts of his invention.

The War Office contacted Matthews in February 1924 to request a demonstration of his ray. Matthews did not reply to them but spoke to journalists and demonstrated the ray to a Star reporter by igniting gunpowder from a distance. He still refused to say how the ray actually worked, just insisted that it did. When the British government still refused to rush to buy his ideas, he announced that he had an offer from France.[4]

The Air Ministry was wary, partially because of previous bad experiences with would-be inventors. Matthews was invited back to London to demonstrate his ray on 26 April to the armed forces. In Matthews's laboratory they saw how his ray switched on a light bulb and cut off a motor. He failed to convince the officials, who also suspected trickery or a confidence game. When the British Admiralty requested further demonstration, Matthews refused to give it.

On 27 May 1924, the High Court in London granted an injunction to Matthew's investors that forbade him from selling the rights to the death ray. When Major Wimperis arrived at Matthews's laboratory to negotiate a new deal, Matthews had already flown to Paris. Matthews's backers appeared on the scene as well and then rushed to Croydon airport to stop him, but were too late.

Public furore attracted interest of various other would-be inventors who wanted to demonstrate their own death rays to the War Office. None of them were convincing. On 28 May Commander Kenworthy asked in the House of Commons what the government intended to do to stop Matthews from selling the ray to a foreign power.[5] The Under Secretary for Air answered that Matthews was not willing to let them investigate the ray to their satisfaction. A government representative also stated that one ministry official had stood before the ray and survived. Newspapers continued to root for Matthews.

The government required that Matthews would use the ray to stop a petrol motorcycle engine in the conditions that would satisfy the Air Ministry. He would receive £1000 and further consideration. From France, Matthews answered that he was not willing to give any proof of that kind and that he already had eight bids to choose from. He also claimed that he had lost sight in his left eye because of his experiments. His involvement with his French backer Eugene Royer aroused further suspicions in Britain.

Sir Samuel Instone and his brother Theodore offered Matthews a huge salary if he would keep the ray in Britain and demonstrate that it actually worked. Matthews refused again – he did not want to give any proof that the ray worked as he claimed it would.

Matthews returned to London 1 June 1924 and gave an interview to the Sunday Express. He claimed that he had a deal with Royer. The press again took his side. The only demonstration Matthews was willing to give was to make a Pathé film The Death Ray to propagate his ideas to his own satisfaction. The device in the movie bore no resemblance to the one government officials had seen.[6][7]

In July 1924, Matthews left for the USA to market his invention. When he was offered $25,000 to demonstrate his beam to the Radio World Fair at Madison Square Garden, he again refused and claimed, without foundation, that he was not permitted to demonstrate it outside England. US scientists were not impressed. One Professor Woods offered to stand in front of the death ray device to demonstrate his disbelief. Regardless, when Matthews returned to Britain, he claimed that the USA had bought his ray but refused to say who had done it and for how much. Matthews moved to the USA and began to work for Warner Bros.

On 24 December 1930 Matthews was back in England with his new creation – a Sky Projector that projected pictures onto clouds. He demonstrated it in Hampstead by projecting an angel, the message "Happy Christmas" and a reportedly "accurate" clock face. He demonstrated it again in New York. This invention was not successful either, and by 1931 he faced bankruptcy. He had used most of his investors' money for living in expensive hotels.

In 1934 Matthews had a new set of investors and relocated to Tor Clawdd, Betws, South Wales. He built a fortified laboratory and his own airfield. In 1935 he claimed that he worked on aerial mines and in 1937 that he had invented a system for detecting submarines. In 1938 he married Ganna Walska, a Polish-American opera singer, perfumer, and feminist, whose four previous husbands had owned fortunes totalling $125,000,000.[9]

Later he propagated the idea of the "stratoplane" and joined the British Interplanetary Society. His reputation preceded him and the British Government was no longer interested in his ideas.

H. Grindell-Matthews, "Diabolic Rays", Popular Radio magazine, New York, Vol. 6, No. 4, August 1924, p. 149-154. Article in which Grindell-Matthews gives a limited description of the rays and what they can do.

^"Grindell 'Death Ray' Matthews". Fortean Times. October 2003. Retrieved 21 July 2007. By any standards Harry Grindell Matthews led a remarkable life. Born in 1880 at Winterbourne in Gloucestershire, he was educated at the Merchant Venturer's School in Bristol before training as an electrical engineer. During the Boer War he enlisted in the Baden-Powell South African Constabulary and was wounded twice. On his return to Britain he pursued his interest in the burgeoning electrical sciences on the estate of Lord de la Warr at Bexhill-on-Sea. There he displayed a natural aptitude for "thinking outside the box" and began to first visualise and later produce a remarkable series of inventions.

^"Invisible Death". Time (magazine). 21 April 1924. Retrieved 21 July 2007. H. Grindell-Matthews, inventor of a method of controlling motorboats at sea by wireless, for which the British Government awarded him $125,000, has perfected a principle by which aeroplane or other engines can be stopped in full operation through an invisible ray.

^"Diabolical Rays". Time (magazine). 9 June 1924. Retrieved 21 July 2007. Harry Grindell Matthews plunged deeper into an orgy of mysterious dickering with prospective purchasers of his invisible "death ray."

^"Grindell-Mathews". Time (magazine). 25 August 1924. Retrieved 21 July 2007. A beam of light shoots from a projector. It seeks out a mouse in its cage. The mouse blinks, surprised, into the glow. A switch is turned. Terrible energy flies along the beam. The mouse jumps into the air, quivers, is dead. So, in the future, Prof. Grindell-film such prophetic visions—the death ray will sweep whole armies into oblivion, whole cities into bleak, smoldering ruins, explode bombs in midair, blow up ammunition dumps from great distances; in a word, make existence for those who do not possess its mysterious secret impossible, and, so he says, end war.

^"Luminaphone". Time (magazine). 23 November 1925. Retrieved 21 July 2007. Last week Harry Grindell-Matthews, British inventor of the "death-ray" demonstrated certain devices with which he had turned theoretical flippancies of the dilettanti into mechanical realism. It is of course an impossibility to rearrange the human nervous system so that one kind of sense impression is substituted for another, but it is quite within the scope of science to turn light into music, sound into color. His instrument, called the "luminaphone," releases light from a series of searchlights to strike through a pattern of holes on revolving disks.

^"Married". Time (magazine). 7 February 1938. Retrieved 21 July 2007. Ganna Walska d'Eighnhorn Fraenkel Cochran McCormick, 45, Polish-American opera singer, perfumer, feminist, whose four previous husbands had owned fortunes totaling $125,000,000; to Harry Grindell-Matthews, 57, inventor of the "death ray," which knocked out a cow 200 yards distant at its first British War Office tests; in London. The bride went on her honeymoon alone, while the investor rushed to his Clydach, Wales laboratory (fenced with electrified wire) to perfect an aerial torpedo.